The present invention relates generally to apparatus for forming tubular conduit and the like, and more particularly to a novel method and apparatus for forming a generally straight spirally wound tube from metallic strip material having an uninterrupted helical welded seam therealong, the method and apparatus having particular application in the forming of relatively small diameter tubing from either flat or fluted strip stock.
Apparatus for forming spiral tubing from either flat or longitudinally fluted metallic strip material by forming the strip into a helically wound tubular configuration and securing helical abutting edges into a continuous seam are generally known. See for example, U.S. Pat. No. 3,601,570. The apparatus disclosed in U.S. Pat. No. 3,601,570 is relatively complex and is particularly suited for making relatively large diameter metallic pipe having either a smooth or corrugated helical wall by continuously welding abutting edges of the helically coiled strip to form a continuous butt-welded seam. Other spiral tube forming apparatus are known which form the helical seam by mechanically interlocking the juxtaposed edges of the metallic strip as it is formed into a generally cylindrical tube.
In the forming of spiral tubes from relatively thin metallic strip material which is either flat or has longitudinal flutes formed therealong, and wherein the diameter of the formed tube is to be relatively small, such as in the order of 1/2 to 2 inches, and the helical edges are to be secured by high speed welding utilizing a narrow welding arc, control of the relation between juxtaposed longitudinal edges of the strip during forming into a spiral tube is extremely important if uninterrupted leak-proof joints are to be achieved. Further, since the speed at which welding can be accomplished in the forming of spiral tubes is the primary factor in determining the production rate obtainiable from a spiral tube forming machine, high speed welding processes such laser welding and high frequency induction welding should be compatable with the tube forming machine.
In forming spiral tubes from relatively thin metallic strip material by feeding the strip material through a generally cylindrical bore in a forming die, the diameter of the tubing is a function of the internal diameter of the cylindrical bore and the angle between the axis of the bore and the direction of movement of the strip material as it is fed into the forming die. This angle determines the helix angle of the formed tube. As the strip material is formed into a helical tube within the forming die, the strip material undergoes a strain such that the spiral strip has a tendency to open up or spring back as it exits from the forming die. With the spiral tube being welded at a location downstream from the exit end of the forming die, it is particularly important that means be provided to prevent springback or opening of the spirally wound tube as it exits from the forming die and also enable control of the relationship between the juxtaposed edges of the wound strip so as to obtain an uninterrupted leak-proof weld seam.